The ability to examine the connectivity and functions of specific brain nuclei and specific neuron types is critical for understanding how the brain works at both microcircuit and mesoscale levels. Therefore tools that enable targeting of selected types of neurons and/or groups of neurons within the neural system, especially tools that are easily applicable and can be used in many different species (model organisms) are important for the neuroscience community. In mouse, a small number of transgenic lines have been generated that express GFP or Cre recombinase in certain selected neuron types or brain areas, whereas such tools are essential but not available in other organism such as rat and non-human primates. In preliminary studies, we found that non- surgical intra-ventricular injection of AAV vectors carrying GFP or Cre under the control of several ultra conserved enhancer elements gave rise to a different yet specific and reproducible pattern of expression in the peripheral and central nervous system. Here we propose to generate region-, neuron type-specific, and activity-dependent AAV vectors based on conserved enhancers. The ease of intra-ventricular injection and the flexibility of AAV vectors will enable a wide range of applications without the need of generating multiple transgenic lines, and the conserved nature of enhancers allow these vectors to be readily applicable in multiple species. Specifically, we will generate a suite of AAV vectors expressing GFP, Cre, or other transgenes under the control of different enhancers, including (1) vertebrate ultra conserved enhancers identified through comparative genomic studies, (2) enhancers to drive expression in inhibitory interneurons, and (3) conserved neuronal activity-dependent enhancers. These vectors will first be screened in mouse using intra-ventricular injections. Subset of AAV vectors with desired expression patterns will be tested in rat and marmoset using the same intra-ventricular injections. We expect in the three-year funding period to generate multiple AAV vectors that enable circuit- and neuron-type-specific investigations and are applicable in multiple species.